Usual magnetoresistive memory devices (see e.g. Z. Wang, Y. Nakamura, J. of Magnetism and Magnetic Materials 159(1996), 233) such as the GMR memory cell are composed of at least two ferromagnetic layers separated by a non-ferromagnetic layer. For the writing procedure each cell has a word line. By applying a current to the word line, the magnetization of the upper layer can be switched by the induced magnetic field. In order to keep the bottom layer pinned in a given direction, an intermediate non-ferromagnetic layer is inserted between the two ferromagnetic layers allowing magnetic exchange coupling to be avoided. The two magnetic configurations, corresponding to the binary information "1" or "0", are "1" when both layers are parallel and "0" when the layers are antiparallel. In addition to the word line, for writing, a separate sense line contacting the layers is provided in order to read the magnetic configuration, "1" (parallel=low resistivity) or "0" (antiparallel=high resistivity).
In such known devices, the write and read currents flow in separate word and sense lines, leading to constraints in the architecture, which must be adapted to the structure of the recording unit or logical processing (address of the memory cell, amplification, drivers, etc.), and necessitating also a high word current. Moreover, such known structures are difficult to scale down.
In order to avoid the presence of two line nets (word line and sense line) and to simplify the memory cell architecture, some alternative solutions have been proposed.
One example is a multilayer device described in U.S. Pat. No. 5,695,864, which avoids the need to induce a local field by the word line. This device has a special multilayer configuration and requires a special write stylus.
A second example is a specific spin-valve memory architecture proposed by L. V. Melo et al. (IEEE Trans. on Magnetics 33(1997), 3295) wherein memory bits each made of a spin-valve sandwich stripe are arranged in a matrix. Each bit is addressed with column and row contacts which are connected as a single line via the spin-valve stripe, making a separate word line unnecessary. However, this proposal relates solely to a four-layer system with a specific spin-valve architecture wherein information is stored in a pinned layer perpendicular to the current in the strip. A "1" or "0" is written depending on the direction of the current. With this geometry, demagnetizing fields and high writing currents (of the order of 20 mA) have to be taken into account when scaling down in dimensions. Given that a typical spin-valve stripe measures 4 .mu.m.times.1 .mu.m, there is no prospect of scaling down by several orders of magnitude to achieve a very high areal density.